From my friend — and canon lawyer working at Sacred Heart Major Seminary in Detroit — Dr. Ed Peters:
A saying in canon law runs thus: "There's no law against it till somebody does it." Well, somebody's doing a new "it", so maybe it's time for a new law. Some canonical thoughts on the priest who brings his doggies to Mass at: http://www.canonlaw.info/2006/09/canine-canons.html